lim(x趋于无穷）根号下x平方加一减去根号下x平方减一,
=lim(x--->无穷）(根号（x^2+1)-根号（x^2-1))
=lim(x--->无穷）((x^2+1)-(x^2-1))/(根号（x^2+1)+根号（x^2-1))
=lim(x--->无穷）2/(根号（x^2+1)+根号（x^2-1))
=0